The present application relates to unloader valves of air compressors, and is particularly directed to an autolift-resistant piston assembly for an unloader valve of an air compressor such as a vehicle air compressor used in an air braking system of a heavy vehicle like a truck.
A truck air braking system includes a vehicle air compressor which builds air pressure for the air braking system. A governor controls system air pressure between a preset maximum pressure level and a minimum pressure level by monitoring the air pressure in a supply reservoir. When the supply reservoir air pressure becomes greater than that of a preset “cut-out” setting of the governor, the governor controls the compressor to stop the compressor from building air. As the supply reservoir air pressure drops to a preset “cut-in” setting of the governor, the governor returns the compressor back to building air.
The vehicle air compressor is typically a reciprocating air compressor and runs continuously. The compressor runs either in a loaded mode or an unloaded mode. When the compressor is running in the loaded mode, compressed air is delivered to the air braking system. When the compressor is running in the unloaded mode, compressed air is directed to an alternate place which is other than the air braking system. Also, when the compressor is running in the unloaded mode, an unloader valve releases pressurized air building up inside the compressor to reduce the pressurized air in the compressor, which in turn reduces the load on the device driving the compressor. This minimizes power consumption during operation of the compressor in the unloaded mode.
One type of unloader valve includes an unloader piston having an interior chamber into which compressed air flows during a compression stroke of the air compressor. More specifically, compressed air flows from cylinder bore of the compressor into the interior chamber of the unloader piston to move the unloader piston from an unloaded position to a loaded position in response to an air signal from the governor. The movement of compressed air flow from the cylinder bore of the compressor into the interior chamber of the unloader piston takes a finite amount of time, resulting in a pressure differential between the interior chamber of the unloader piston and the cylinder bore of the compressor. In some unloader valves, springs and balancing air forces are needed to hold the unloader piston in the loaded position to minimize any movement not initiated by the air signal from the governor.
The pressure differential between the interior chamber of the unloader piston and the cylinder bore of the compressor results in an opening force which opposes the biasing force of a coil spring. The biasing force of the coil spring biases the unloader piston toward the loaded position. Thus, the opening force opposing the biasing force of the coil spring pushes the unloader piston away from the loaded position towards the unloaded position.
An unloader piston autolifts when the cylinder pressure provides greater force upward than the spring force downward. Autolifting of the unloader piston (which can occur up to 50 times per second) results in rapid wear of O-rings which are disposed on the unloader piston. Moreover, autolifting of the unloader piston results in substantial reduction of compressed air flow to the interior chamber of the unloader piston.
One way to reduce autolifting of the unloader piston is to equalize pressure (i.e., eliminate the pressure differential) between the interior chamber of the unloader piston and the cylinder bore of the compressor by increasing the diameter of a breathing hole (a.k.a. a piston breathing hole) which communicates between the interior chamber of the unloader piston and the cylinder bore of the compressor. However, the increase in diameter of the breathing hole reduces compressor efficiency. It would be desirable to provide an unloader valve which overcomes drawbacks of known unloader valves.